Water-selective and multifunctional aquaporins from Lotus japonicus nodules

By using reverse transcriptase-polymerase chain reaction, two cDNAs were isolated that encode major intrinsic membrane proteins (MIPs) that are expressed in nitrogen-fixing root nodules of Lotus japonicus. Lotus intrinsic membrane protein 1 (LIMP 1) is expressed at high levels in both nodule and root tissues and shows highest sequence similarity to members of the tonoplast intrinsic protein (TIP) subfamily of plant MIPs. Functional analysis of LIMP 1 by expression in Xenopus laevis oocytes show that it is a water-specific aquaporin. In contrast, LIMP 2 shows the highest sequence similarity to soybean nodulin 26 (67.8% amino acid sequence identity). LIMP 2 is also a nodulin, showing expression only in mature nitrogen fixing nodules of L. japonicus. LIMP 2 is a multifunctional aquaglyceroporin, and displays the ability to flux both water as well as glycerol upon expression in Xenopus oocytes. Additionally, the carboxyl terminal region of LIMP 2 has a conserved phosphorylation motif that is phosphorylated by a calmodulin-like domain protein kinase. Overall, the data show that L. japonicus nodules contain two structurally and functionally distinct MIP proteins: one (LIMP 2) which appears to be the nodulin 26 ortholog of L. japonicus and another (LIMP 1) which appears to be a member of the TIP subfamily.